Superhydrophobic composite films produced on various substrates

Hydrophilic silica (SiO₂) nanoparticles were dispersed in solutions of poly(methyl methacrylate) (PMMA) and in solutions of a commercial poly(alkyl siloxane) (Rhodorsil 224), and the suspensions were sprayed on glass surfaces. The effect of the particle concentration on the hydrophobic character of PMMA-SiO₂ and Rhodorsil-SiO₂ films was investigated and showed the following: (i) Static contact angles (θ_S), measured on surfaces that were prepared from dilute dispersions (particle concentration < 1% w/v), increase rapidly with particle concentration and reach maximum values (154 and 164° for PMMA-SiO₂ and siloxane-SiO₂, respectively). Further increases in particle concentration do not have any effect on Θ_S. (ii) The effect of particle concentration on the contact angle hysteresis (θ_A -θ_R) is more complicated: as the particle concentration increases, we first notice an increase in hysteresis, which then decreases and finally becomes constant at elevated particle concentrations. The lowest θ_A - θ_R values were 5° for PMMA-SiO₂ and 3° for siloxane-SiO₂, respectively, (iii) SEM and AFM images show that a two-length-scale hierarchical structure is formed on the surface of the superhydrophobic films. It is demonstrated that superhydrophobicity can be achieved using various hydrophilic nanoparticles (alumina and tin oxide nanoparticles were successfully tested) and that the substrate has almost no effect on the hydrophobic character of the applied coatings, which were produced on silicon, concrete, aluminum, silk, wood, marble, and of course glass. The results are discussed in light of Wenzel and Cassie-Baxter models.